LLC or not?
An LLC is a limited liability company, meaning that if something happens and you get sued (and loose) the only assets that can be used to satisfy the judgement are the assets of the LLC, not your own. Currently, being a sole proprietor you are personally liable for all potential damages caused by your business. So if your business is in fact growing it might be a good idea to organize it under an LLC.
As far as cost goes, yes, $184 is a basic LLC formation package we currently offer (as of Feb 2012) that would get your LLC formed. Keep in mind that you might need a new EIN.